The present invention relates to a data symbol reader for reading information encoded in a data symbol, for example, two-dimensional tessellated data symbols.
Bar codes and associated devices for reading bar code information are widely used for applications such as point-of-sale (POS) systems. However, most common bar codes and readers use the familiar one-dimensional format, in which information is encoded in a one-dimensional arrangement of parallel bars. A one-dimensional format data symbol can hold only a small amount of information.
Two dimensional symbols using a binary-encoded mosaic (or "tessellated") rectangular pattern, typically black and white, hold more information. Data symbol readers for reading the tessellated symbols use either an area image sensor or a line image sensor. With a line sensor, the reading device and data symbol are moved relative to each other for auxiliary scanning, and the line sensor repetitively main scans lines of the data symbol to compose an area image. With an area sensor (usually an area CCD) an image of the data symbol pattern is captured in two dimensions simultaneously. Reading devices which use an image sensing device (i.e., an area image sensor) do not require relative movement between the reading unit and the data symbol for sub-scanning, and are thus able to read data symbols in a shorter time.
A stationary data symbol reader may be used for reading a moving data symbol. One example of an appropriate application is reading data symbols borne by products moving along an inspection or conveying line in a factory, either automatically or by hand.
When the data symbol reader is stationary, but the symbol itself moves, the moving speed of the symbol varies. If automatic, the speed will vary between conveying devices. If an operator manually moves the item bearing the symbol past the symbol reader, the speed will constantly vary. The conventional symbol reader cannot obtain the moving speed of a data symbol to be read.
In a case where a moving data symbol is read by a conventional data symbol reader having an image sensing device, it is very difficult to accurately set an exposure (exposure time, light amount) suiting the moving speed of the data symbol. If the exposure time is too long, the image blurs and becomes unreadable. If the exposure time is too short, sufficient imaging information is not extracted, (for example, insufficient levels of contrast for thresholding,) leading to reading errors.